On the nature of mixed potential in a DMFC

A 1D + 1D model of a direct methanol fuel cell (DMFC) is developed. Analytical solution for the case of equal oxygen and methanol stoichiometries l is obtained. The model predicts a new effect: formation of a narrow zone of local current (bridge), which "short-circuits" cell electrodes at vanishingly small current in the load. The bridge arises only under nonzero flux of methanol through the membrane. This effect explains dramatic decrease of DMFC open-circuit voltage known as mixed potential. The expression for voltage loss at open-circuit E-0 is derived; E-0 increases with the rate of crossover and tends to zero with the growth of l. Numerical estimate for typical cell gives E-0 similar or equal to 500- 700 mV, which agrees with the experimental data. The model suggests a method for measuring mean crossover flux through the membrane and indicates conditions for cell operation with zero crossover, regardless of the transport properties of the membrane (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.1905963] All rights reserved.